Field of the Invention
The present invention relates generally to the field of manufacturing forged aluminum connecting rods, and more particularly to an improved high performance aluminum connecting rod and a method for manufacturing such a connecting rod which is capable of carrying particularly high compressive loads with a substantially reduced deformation resulting from such high compressive loads, the improved high performance aluminum connecting rod of the present invention finding particular application in the field of high performance racing engines.
Internal combustion engines used in automobiles typically have pistons which reciprocate in cylinders contained in an engine block, with the pistons driving connecting rods which in turn drive a single geometrically complex crankshaft, causing it to rotate within the engine block. The pistons are cylindrical and are hollowed out at the end facing the crankshaft. Wrist pins extending diametrically through the pistons near the open end thereof are used to connect each piston to one end of a connecting rod which has a cylindrical wrist pin bore located therein, which end is referred to as the pin end of the connecting rod.
The other end of each connecting rod is connected to the crankshaft at a particular cylindrical bearing location on the crankshaft. This end of the connecting rod, which is referred to as the crank end, also has a cylindrical bearing housing bore located therein. Typically, an annular bearing is located between the bearing housing bore at the crank end of the connecting rod and the cylindrical bearing location on the crankshaft.
The crank end of the connecting rod is split into two segments, with the split being defined by a plane which lies at an angle to the longitudinal axis of the connecting rod (the axis extending between the pin end and the crank end of the connecting rod), which plane divides the bearing housing bore at the crank end of the connecting rod in half. The end of the main segment of the connecting rod (the segment which includes the pin end) which is opposite the pin end is referred to as the fork of the connecting rod, since it includes half of the bearing housing bore at the crank end of the connecting rod.
The smaller segment of the connecting rod is referred to as the cap or the bearing cap, and it includes the other half of the bearing housing bore at the crank end of the connecting rod. The cap typically is retained in position on the fork by two bolts located on opposite sides of the cap and fork. The bolts extend into holes located in the cap, and are screwed into threaded apertures located in the fork.
As is generally well known in the art, the development of the reciprocating internal combustion engine has resulted in ever-smaller engines which are capable of generating an ever-increasing amount of horsepower, at ever-higher RPM's. It will be appreciated by those skilled in the art that with the increasing power has come increased stresses which are placed on the various parts of the engine, including increased stresses placed on the connecting rods and bearings. These advancements in engine technology have resulted in a demand for connecting rods which are both lighter and stronger than previous generations of connecting rods.
Nowhere is this demand for improved connecting rod strength and lightness more important than in the field of auto racing, including gas, alcohol, and nitro engines which are used in drag racing. These engines are subjected to incredible loads, and this is particularly true with respect to the ultra high combustion pressure loads that the connecting rods are subjected to. The loads are compressive in nature, and whenever the yield strength of the connecting rods is exceeded, they will be damaged or destroyed. Typical damage which can result from these stresses includes bending of the connecting rod beam (the portion of the connecting rod extending between the pin end and the fork), elongation of the wrist pin bore, and deformation of the bearing housing bore, particularly in the cap of the connecting rod.
The materials which are used to make high performance connecting rods are almost exclusively high tech aluminum alloys, which possess both relatively light weight and the requisite high degree of material strength. Aluminum alloys have been used for high performance engines such as those used in drag racing for thirty years, with the first alloy being used being 2014-T6 aluminum alloy, which has a compressive yield strength of 58 ksi. More recently, 7075-T6 aluminum alloy has been used, which has a substantially higher compressive yield strength of 72 ksi. Neither of these alloys currently possesses sufficient compressive yield strength to withstand the compression loads of current drag racing engine technology.
In addition, the manufacture of high performance aluminum connecting rods from aluminum material must be of a nature which enhances, and in no way reduces, the material strength characteristics of the aluminum material. To understand this, it is necessary to briefly describe the manufacture of connecting rods. There are three primary methods of manufacturing connecting rods: casting, powder metal manufacturing rolled plate stock, and foregoing. Each of these methods will be briefly described below.
Casting involves heating the metal alloy which will be used until it is molten, and pouring the molten metal alloy into "sprews" which are connected to "runners" which channel the molten metal alloy into a multiple cavity mold. The molten metal alloy fills the mold, and vents through "risers." After cooling, the metal casting is separated from the mold, the sprews, runners, and risers are removed, and the casting is machined.
Casting is a relatively low-cost manufacturing process, and results in a product with tight dimensional tolerances. Unfortunately, casting has several disadvantages, including relatively low strength characteristics, an absence of grain flow strength, and possible porosity of the cast article. These negative characteristics make casting an unacceptable manufacturing process for high performance aluminum connecting rods.
Powder metal manufacturing involves placing a mixture of metal powder into a preformed die, and then compressing the powder in the die with a compacting press to mechanically bond the metal powder into a preform. The preform is then sintered to chemically bond the powder in the preform, and the preform is then heated and forged in a press. The resulting article is then machined to finish it.
Powder metal manufacturing results in high tolerances and uniform material composition of the finished article. Unfortunately, powder metal manufacturing is a relatively high cost operation, has a lack of grain flow strength, and results in a product which has relatively low tensile strength and impact strength. These negative characteristics make powder metal manufacturing an unacceptable manufacturing process for high performance aluminum connecting rods.
The rolled plate stock approach includes machining to form a billet connecting rod. However, the rod has relatively low strength and no grain flow strength.
Forging involves heating a piece of bar stock or rod stock (referred to as a forge slug) to a forging temperature, placing the heated forge slug into a two piece forging die defining a cavity within, and forging the forge slug using a forging press or forging hammers with multiple impacts. Excess material results in flashing extending between the two dies. This flashing is removed by placing the forging into a trim die, which is placed in a punch press to trim the flashing from the forged article. The part is then machined to finished dimensions.
Forging can be somewhat labor intensive due to the amount of machining which is necessary to produce the finished article, but it produces a product which has strength characteristics which are superior to those produced by casting rolled plate stock or powder metal manufacturing. This results from the fact that the natural grain of rolled metal stock is maintained in the finished product. The natural grain extends along the full length of the forged rod, from one end where it is expanded. The natural grain is not truncated, as is the case with a billet. As such, it will be apparent to those skilled in the art that forging is the preferred method of manufacture for high performance aluminum connecting rods.
It is accordingly the primary objective of the present invention that it provide an improved high performance aluminum connecting rod made of an improved material capable of withstanding the high compressive loads of current drag racing technology. The material used by the improved high performance aluminum connecting rod of the present invention must have lightweight construction which is comparable to presently known high performance aluminum materials such as 7075-T6, while possessing substantially enhanced compression yield strength characteristics as compared to such presently known materials. In addition, this improved material must retain or enhance all of the other favorable material characteristics of such presently known materials.
The improved high performance aluminum connecting rod of the present invention must be susceptible of manufacture by conventional forging techniques, with the forged part being readily machinable to the required finished dimensions. It is also necessary that the high performance characteristics of the aluminum material used to make the improved high performance aluminum connecting rod of the present invention not be adversely affected in the forging or machining operations. Most importantly, it is an objective of the improved high performance aluminum connecting rod of the present invention that it have sufficiently improved compression yield strength so as to hold both its dimensional length and the dimensional roundness of the wrist pin bore and the bearing housing bore.
The improved high performance aluminum connecting rod of the present invention must be of a construction which is both durable and long lasting, and it must remain within dimensional specifications throughout an extended operating lifetime. In order to enhance the market appeal of the present invention, it should also be of relatively inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives of the improved high performance aluminum connecting rod of the present invention be achieved without incurring any substantial relative disadvantage.